Polymorphic Layered MoTe₂ from Semiconductor, Topological Insulator, to Weyl Semimetal

Large size (∼2 cm) single crystals of layered MoTe₂ in both 2<i>H-</i> and 1<i>T</i>′-types were synthsized using TeBr₄ as the source of Br₂ transport agent in chemical vapor transport growth. The crystal structures of the as-grown single crystals were fully characterized by X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, scanning tunneling microscopy (STM), and electrical resistivity (ρ) measurements. The resistivity ρ­(<i>T</i>), magnetic susceptibility χ­(<i>T</i>), and heat capacity <i>C</i>_p(<i>T</i>) measurement results reveal a first order structural phase transition near ∼240 K for 1<i>T</i>′-MoTe₂, which has been identified to be the orthorhombic <i>T</i>d-phase of MoTe₂ as a candidate of Weyl semimetal. The STM study revealed different local defect geometries found on the surface of <i>2H</i>- and <i>T</i>d-types of MoTe₆ units in trigonal prismatic and distorted octahedral coordination, respectively

Proximity-Induced Superconductivity and Quantum Interference in Topological Crystalline Insulator SnTe Thin-Film Devices

Topological crystalline insulators represent a new state of matter, in which the electronic transport is governed by mirror-symmetry protected Dirac surface states. Due to the helical spin-polarization of these surface states, the proximity of topological crystalline matter to a nearby superconductor is predicted to induce unconventional superconductivity and, thus, to host Majorana physics. We report on the preparation and characterization of Nb-based superconducting quantum interference devices patterned on top of topological crystalline insulator SnTe thin films. The SnTe films show weak anti-localization, and the weak links of the superconducting quantum interference devices (SQUID) exhibit fully gapped proximity-induced superconductivity. Both properties give a coinciding coherence length of 120 nm. The SQUID oscillations induced by a magnetic field show 2π periodicity, possibly dominated by the bulk conductivity